The invention relates to a system for extracting and decontaminating groundwater and utilizing geothermal heat and also to the use of the system according to the invention.
The utilization of geothermal heat is regarded as belonging to the regenerative energies. The temperature in the soil rises with increasing depth. Below 20 meters this increase in temperature is no longer dependent on seasons or climate, but essentially depends on geological and geothermal conditions. For this reason, the utilization of geothermal heat for energy production is a good option in many regions because emission-neutral, especially with respect to CO2 emissions, and safe production of energy is possible in this way.
Various systems and processes for the utilization of geothermal heat are known from the prior art. One current method of utilization is a geothermal heat probe, such as disclosed in DE 29 35 832 A1. Therein, a U-pipe is introduced in a borehole in the soil. A liquid circulating in the tube absorbs heat from the environment in the depths, which heat is subsequently utilized. A pump is provided to support circulation. This solution is disadvantageous in that the efficiency of the system is limited for constructional reasons and antifreeze agents must be added which involve a risk of pollution.
Other alternatives subsumed under geothermal energy are geothermal heat collectors, which are remarkable for their horizontal laying close to the surface, and brine circulating therein, as well as pumpless geothermal heat collectors with direct evaporation of a refrigerant.
Groundwater offers another possible way of utilizing geothermal heat by conveying the groundwater through an extraction well to the surface where heat is withdrawn therefrom by means of a heat pump. The water is subsequently fed back into the aquifer through an injection well. This solution is disadvantageous in that separate wells are required for water extraction and injection.
The chemical and physical parameters of water withdrawn from the extraction well frequently differ from those of water in the injection wells just a few meters away so that chemical reactions and precipitation reactions take place, blocking the well in the long run. In addition, the pressure differences occurring during above-ground pumping involve the risk of out-gassing of dissolved gases and precipitation reactions associated therewith.
EP 0 386 176 B1 discloses a system for exchanging energy between the soil and an energy exchanger via a combination of a forward pipe with a pump in the borehole and a feedback pipe. The borehole is provided with a porous filling, and water is introduced into the borehole through the forward pipe to reach the feed-back pipes through the porous filling. The feedback pipes are provided with a combination of transverse seals and through-openings in the direction of the porous filling so that the water, when conveyed to the surface, is always forced to leave the feedback pipe. The special configuration of the feedback pipes is intended to increase the heat absorption of the water. This solution is disadvantageous in that implementation thereof with sufficient efficiency is only possible over long lengths.
EP 0 755 497 B1 discloses a system for extracting geothermal heat, wherein water is introduced down to the bottom of the bore in the outer region of the borehole. A shroud pipe is arranged at a defined distance to the bottom of the bore, which pipe has a pump in the lower region thereof, the pump being intended to convey water to the soil surface. The region of the bore between the outlet opening of the water-supplying pipes and the lateral opening of the shroud pipe is provided with a porous filling. Although the inventive measures of EP 0 755 497 B1 are intended to take up preferably warm water from the lower region of the bore, the existence of a hydraulic connection between supplying and discharging pipes is disadvantageous, so that preferably cold water is conveyed to the surface which has previously been introduced into the borehole. As a result, the efficiency decreases considerably.
Solutions known from the prior art preferably use separate pipes in the borehole to extract and return the groundwater, as well as a separate heat exchanger in the form of a separate system. As a result, the systems known from the prior art are complex and cause high expenses due to the requirement of larger bore diameters.
JP 58024762 describes a method for extracting geothermal heat using a main pipe provided with through-openings upstream and downstream of a transverse seal. Therein, withdrawal from a groundwater-bearing layer and introduction into a hydraulically separated, different aquifer have been depicted as being essential and fundamentally necessary. This results in mixing of different groundwaters usually having different chemical and physical water qualities, e.g. bearing freshwater and saltwater, which may give rise to precipitation reactions and blocking of the well filter sections in the long run. Also, mixing of different groundwaters is prohibited in most regions for groundwater protective and ecological reasons and involves problems in regions with groundwater utilization for drinking water production.
In a large number of countries, a technical teaching such as the one in JP 58 064 762 is therefore not permissible because it involves mixing of chemically different waters, which gives rise to problematic chemical reactions and precipitations. Apart from the resulting pollution of the environment, this also implies that the corresponding wells would be subject to damage after a certain time of use, and such damage would accumulate during the course of time, thereby resulting in total failure of the system.
Furthermore, wells for the utilization of geothermal heat have been described in the prior art, e.g. according to DE 271 54 99, DE 28 50 865, or CH 65 31 20.